Descon Cast Away

Scratch - Cast Away {Scratch}

Contributed by Rick Dickinson

Manufacturer: Scratch
Contributed by - Rick "OddRoc" Dickinson


A few months ago, I was involved in a nasty little sledding accident -- some kid crashed into me from behind, breaking my tibia.While in my cast, I started thinking about the DesCon possibilities....

My DesCon entry is named "Cast Away", and is built from my cast-off fiberglass leg cast.It uses a 38mm motor mount and rail guides, and uses Lexan fins to preserve the overall appearance.

This is a picture of me actually wearing the cast that would later become "Cast Away".The cat in the picture is Sheba, one of our 3 cats.

To construct a similar rocket, you will need:

  • 1 ea. fiberglass leg cast, including foot and ankle
  • 3 ea. Lexan fins
  • 1 ea. 38mm LOC motor mount tube
  • 1 ea. 3.9" diameter LOC tubing, 10" or so long
  • 1 ea. 3.9" coupler tube, 6" long
  • 1 ea. 1/4" plywood bulkhead, sixed to fit inside the 3.9" tubing
  • Shock Cord (tubular nylon)
  • Parachute
  • Maybe a bit of nose weight
  • Epoxy (I used System Three)
  • Thin and medium CA, with Kicker
  • Fiberglass (for fastening the two halves of the cut-off cast together)
  • NHP 2-part polyurethane foam
  • Devcon Plastic Welder adhesive (possibly -- subject to the results of my testing)

Since my cast was split down the front to accomodate swelling in my leg, and then later cut down the back to remove it, I decided to place one fin in the split running down the front of the foot, making use of the foot itself as part of the fin area.The other two (larger) fins come out of slots cut in the back sides of the cast, allowing the rail guides to be attached at the ankle and the upper thigh.On the rail, the toe will be pointed out, away from the rail.

The first step in construction was rejoining the two halves of the cast.I first pulled out as much of the cotton liner stuck to the inside of the cast as I could, and then tacked the cast back together along the rear split with CA, hitting it with a mist of kicker to cure it instantly.Then, I laid strips of 6 oz. fiberglass down the inside of the split, wetting them down with epoxy using a small paint brush. Neatness here didn't really matter, as the joint would be completely concealed within the cast.

Once that had cured, I attempted to squeeze the front portion of the cast back together to secure it.Unfortunately, it had been initially split the day after it was applied to my leg, so the split had essentially "set" as the cast aged during the two months I wore it. I was able to tack the upper end of the split together, but the lower end, including the foot, is still spread apart, somewhat.This definitely influenced my choice of fin locations....

Fins were cut from a sheet of 1/8" Lexan.The two rear fins were cut as 6" x 12" rectangles, then a 6" x 2" triangle was cut off of the "root edge" to give the fins some rearward sweep, and small 1" x 1" triangles were cut off the exposed corners to make them less likely to be damaged upon landing, and limit the number of sharp edges. The "foot" fin is a 6" x 8" rectangle, with the outside corners similarly trimmed for safety and avoidance of landing damage.

Next, I used a hole saw and a bastard file to create and then enlarge a motor mount hole in the "aft centering ring" -- the heel.Because of the slight bend at the knee, the motor mount tube runs from the heel to a point near the front of the upper thigh.A length of 3.9" tube then sits "over" the end of the 38mm motor mount, angling back towards the center of the upper thigh.A 2" ring of 3.9" tube, with a 1/4" plywood bulkhead and a 6" long coupler tube, makes up my "nosecone".

Once the motor mount tube was test-fit, I eyeballed the slots for the fin attachment.These were cut with a combination of a rotary tool (cheap Harbor Freight Dremel-knockoff) and an X-acto razor saw. I will be experimenting tonight using scrap pieces of Lexan to determine the best adhesive.I will try Devcon Plastic welder as a possible alternative to my usual thickened System Three Epoxy, as I am worried about adhesion of the plastic (Lexan) fins.In any case, I will be sanding with 80 grit, and drilling a number of small holes near the root of the fins, to allow the adhesive of choice to form "rivets" ensuring a good mechanical bond, and will also drill a few 3/8" or so holes to allow the eventual foam application to flow through and around all of the fin locations.

More details and hopefully photos soon.A quickie, not-to-scale sketch I drew up showing (approximately) what I'm building is attached.


2, 3, 4 - These pictures were taken during construction, and show the cast after the installation of the clear Lexan fins (still in their protective white plastic covering), the motor mount and shock cord anchor.Some foaming has already been done to "center" the motor mount tube and fill the fin can area, but the 3.9" tube that will hold the parachute and the "knee cap" (instead of a nose cone) has not been added yet.

As planned, the fins were secured to the motor mount using DevCon Plastic Welder epoxy.Although I had originally planned to drill holes to help give the plastic welder and 2-part foam something to grip, I decided, after a bit of testing with scrap material, that this was completely unnecessary.The fins were, instead, roughed up using 60 grit sandpaper over the entire portion of their surface that extended within the cast.This provided an excellent mechanical bond with the plastic welder and the 2-part foam.


5, 6 - These pictures were taken with the completed rocket loaded onto pad 39A (Bill Seiders' rail launcher) at the monthly ROC launch this past Saturday (5/12/01).You can see that I have trimmed the upper leg back to just above the knee to allow clearance for the rail.The thrust axis of the 38mm motor mount angles through the CG of the cast, which is about halfway up, near the front of the cast.

7   7a

7, 7a - These are two versions of the same picture, showing the smoke trail (I missed the rocket -- it went too fast for my shutter finger) as Cast Away takes to the sky on an I300T-Short Aerotech reload.I've fiddled with the scanner settings on one of them to try to enhance the smoke trail, but I'm not really sure if it did too much good....

Final weight of the rocket (sans engine) was just about 7.75 lbs.Due to the gusty nature of the winds at the launch, and the draggy design of the rocket, I decided to go with a rather more powerful motor than was strictly necessary (Blue Thunder instead of White Lightning, 38/480 casing instead of 38/360 casing), to ensure that I got plenty of speed off the rail to ensure a straight flight.

Well, it was very straight and stable, but the much higher speed led to quite a bit of fin flutter, resulting in two of the Lexan fins breaking during the coast phase of the flight. After burnout, the rocket suddenly started tumbing forward, end over end.It deployed the chute right on schedule, and landed gently on the Lucerne Dry Lake lakebed.

8 - This picture shows Cast Away after deployment of the nylon chute.


This picture shows Cast Away after landing. Note the two broken fins.The "front" fin, which was partially supported by the foot, survived 100% intact.As you can see, both of the fins that broke did so well outside the cast, confirming my decision that the Plastic Welder and the 2-part foam would be more than sufficient to secure and protect the fin roots within the rocket.

Had I had more time for this DesCon, I would have waited until next month's launch in hopes of less windy conditions, and launched CastAway on a smaller White Lightning motor.I believe that the rocket would have still flown fine on an I161W motor, and would not have fluttered the fins off in flight.The 12' long launch rail would have still given the rocket plenty of time to get up to a stable speed, but the lower maximum velocity would have saved the fins.

I will probably repair the rocket, using 1/8" plywood to replace the two broken fins.This should make them much stiffer, and much less prone to flutter.

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